Generally, semiconductor devices comprise active components, such as transistors, formed on a substrate. Any number of interconnect layers may be formed over the substrate connecting the active components to each other and to outside devices. The interconnect layers are typically made of low-k dielectric materials comprising metallic trenches/vias.
As the layers of a device are formed, it is sometimes necessary to planarize the device. For example, the formation of metallic features in the substrate or in a metal layer may cause uneven topography. This uneven topography creates difficulties in the formation of subsequent layers. For example, uneven topography may interfere with the photolithographic process commonly used to form various features in a device. It is, therefore, desirable to planarize the surface of the device after various features or layers are formed.
One commonly used method of planarization is via chemical mechanical polishing (CMP). Typically, CMP involves placing a wafer in a carrier head, wherein the wafer is held in place by a retaining ring. The carrier head and the wafer are then rotated as downward pressure is applied to the wafer against a polishing pad. A chemical solution, referred to as a slurry, is deposited onto the surface of the polishing pad to aid in the planarizing. Ideally, the retaining ring comprises a multitude of grooves to facilitate the even distribution of the slurry over the wafer surface. When retaining rings without any grooves are used during CMP, the resulting wafers tend to suffer topographical unevenness due to irregular slurry disposition. Thus, the surface of a wafer may be planarized using a combination of mechanical (the grinding) and chemical (the slurry) forces.